High Iron Content Oxygen Carrier For Chemical-looping Combustion Of Methane

Chemical-looping combustion technology is a new type of CO₂ capture,oxygen carrier as the core of chemical-looping combustion system is the focus of current research work.Iron-based oxygen carriers have received particular attention due to their low cost and non-polluting properties.However,the high-iron content oxygen carrier is easily sintered and inactivated,it is usually loaded on an inert carrier,but Fe₂O₃ and the inert carrier are prone to phase separation at a high-temperature redox reaction,resulting in the load of active component was reduced（≦50%）.In addition,Fe₂O₃ also reacts with the carrier to form an new metal oxide,which reduced the stability of oxygen carriers.Hexaaluminate has a special layered structure and defect mechanism,which can regulate the diffusion rate of lattice oxygen through charge compensation and ionic radius.In this paper,we utilize the stability at elevated temperature and structural mosaicity of hexaaluminate to prepare high-iron content oxygen carrier,so that Fe³⁺is highly dispersed in hexaaluminate,overcoming the problem of sintering and stability of oxygen carrier due to excessive iron.At the same time,the oxygen carrier was characterized by H₂-TPR,CH₄-TPR,XRD,XPS,etc.And an effective correlation between the performance and microstructure was established,which provided the theoretical basis and design direction for the application of high-iron content oxygen carrier in CLC system.Firstly,BaFe₁₂O₁₉9 oxygen carriers with 60%iron content were prepared by co-precipitation method.Meanwhile,two oxygen carriers with different Fe contents（Ba/Fe=1/2 and 1/48）were prepared to investigate the influence of Fe content on the structure and performance of oxygen carriers.In addition,oxygen carriers were prepared by physical hybrid method in accordance with the ratio of Ba/Fe=1/12 to investigate the influence of the synthesis method on the structure of oxygen carriers.The results show that Ba/Fe=1/2-G OCs forms the pure phase of BaFe₂O₄ spinel,Ba/Fe=1/48-G OCs showed the co-existence of both MP-type hexaaluminate and the Fe₂O₃ phases,Ba/Fe=1/12-G OCs mainly forms the MP-type BaFe₁₂O₁₉9 phase,while Ba/Fe=1/12-P is the coexistence of BaCO₃ and Fe₂O₃ phases.During the CH₄/O₂ redox process,increasing the Fe content can improve the reactivity and stability of oxygen carriers.Forming MP-type structures can significantly improve CH₄ conversion,CO₂selectivity and the amount of O_t.The XPS results revealing that ratio of Fe²⁺/Fe³⁺increased of BaFe₁₂O₁₉9 OCs with the redox cycle going on,indicating that improved the formation of oxygen vacancies and the diffusion rate of lattice oxygen.Therefore,BaFe₁₂O₁₉OCs has the highest reactivity during successive redox cycles.Secondly,to screen and optimize the reaction conditions of BaFe₁₂O₁₉9 oxygen carrier,investigate the influence of different calcination temperature（t=500,700,800,900,1000,1100℃）on the crystal phase composition and reaction properties of BaFe₁₂O₁₉9 OC.At the same time,the effect of reaction temperature on the activity and stability of oxygen carrier was also investigated.It was found that Fe³⁺completely replace Al³⁺reduced the temperature of begins to form MP type phase.The typical MP structure began to form at 700°C.BaFe₁₂O₁₉9 showed highest stability when calcination temperature is 900℃,reduction temperature is 900℃,oxidation temperature is 800℃.After 10^thh cycle,the CH₄ conversion was still 83%,the CO₂ selectivity was 98%,and the O_t was 2.21 mmol/g.The application of La_xBa_1-xFe₁₂O₁₉（x=0,0.2,0.4,0.6）series oxygen carrier in CLC was studied,and the effect of La substitution on the structure and properties of oxygen carrier was investigated.The results showed that La³⁺partially replaced Ba²⁺inhibited the formation of BaFe₂O₄ crystal phase,increases the specific surface area and improves the reactivity and cycle stability of the oxygen carrier.As the amount of La doping increases,the methane conversion and the amount of converted lattice oxygen also increase.Among them,when x=0.6,the best reaction performance was exhibited in 10 cycles.The doping of La³⁺ions reduced the lattice defects of the MP-type structure,enhanced the support of the large cations on the framework of hexaferrite,and promotes the integrity and stability of the MP-type structure.